Four-wheel drive improves your car's grip on the road

By Barry Lake

MOST drivers would have experienced wheel spin and the resultant sudden loss of traction.

While country people quickly become accustomed to wheel spin and loss of grip on surfaces such as gravel roads or wet clay, most urban motorists only suffer wheel spin when it rains.

The basic problem with traction -- the level of grip between the tire and the road surface -- is it is affected by all of the forces transmitted through the tire.

There is a limit to the level of friction between the tire and the road and this, in turn, means there is a limit to the amount of work a tire can handle.

As long as the tire is rolling along the road in a straight line, there is no real problem.

But when you turn the steering wheel, the front tires are being asked to transmit lateral forces to steer the car around the corner.

If the car is traveling too fast or the road surface is slippery, the adhesion limit can be exceeded and the tire will start to slide across the road surface.

This limit can be reached even sooner if the driver pushes down hard on the accelerator pedal while steering the car through a corner.

Sudden and/or excessive application of power can make the driven wheels spin, in which case they lose most of their tractive abilities and will neither drive nor steer effectively.

In the case of a front-wheel drive car, this will cause the vehicle to run wide on a corner -- even straight ahead in the extreme.

With rear-wheel drive, this wheel spin will cause the tail to snap out into a slide or spin.

Putting the drive through all four wheels greatly reduces the tendency to spin the wheels through excessive acceleration.

The load is shared by four wheels rather than just two.

People who have never experienced the difference between two- wheel and all-wheel drive might well be amazed at the difference.

With cars like Porsche's Carrera 4, the various Audi quattro models, Subaru Impreza 4WD and Volvo's V70 AWD Wagon, the level of grip when cornering on slippery surfaces is way beyond that of any equivalent two-wheel drive car.

We speak of on-road 4WD vehicles as having "all-wheel drive", mainly to separate them from rugged all-terrain 4WD vehicles, which often have what is termed "part-time" 4WD systems.

These systems do not have a central differential, which can be an advantage off-road.

But with no means of "splitting" the drive between the front and the rear, they rely on tire slippage on loose or slippery surfaces to avoid mechanical damage.

If used on-road, these part-time 4WD systems can suffer serious mechanical damage due to "wind up" of the transmission caused by a lack of slip between the front and rear axles.

For on-road use, the 4WD system must be fitted with a center differential so drive can be varied between the front and rear, depending on the conditions.

About US$2,000 is a "ball-park" figure on the extra cost of a medium-priced all-wheel drive vehicle over its price if it had drive to two wheels only.

But the advantage is huge if you drive on muddy country roads or you just want that extra margin of safety on wet roads.